Two are believed to have blown themselves up but a third, who was wearing a white shirt and dark hat, remains at large and was the focus of a series of raids in the Schaerbeek​ district of Brussels.

Laachraoui grew up in Schaerbeek and had set up at least one bomb-making factory there in the weeks before Paris. It is feared he was the mastermind behind the latest attack after suggestions the same type of explosive – dubbed "Mother of Satan" – was used.

People comfort each other after being evacuated from Brussels airport, after the explosions. Photo: AP

There were also reports the local train station had been cordoned off but police activity focused on a block of flats on Rue Max Roos. A heavily-armoured police truck blocked off the residential street while a helicopter hovered over head.

Iraqi intelligence sources suggested IS had been planning a terror attack on an airport and train station for three months but its focus was not Brussels originally.

The man who used fake identity documents bearing the name Soutane Kayal was already being sought by the Belgian police as part of the investigation into the November 13 Paris attacks. Photo: AP

He was in a car with Salah Abdeslam, who later became a key fugitive from the Paris attacks, on their way from Budapest to Brussels when they were stopped.

But police waved them on after they convinced officers they were tourists on a trip to Vienna.

Because they were travelling on false identities, police did not link Laachraoui to an international warrant issued for him in March 2014 under his real name.

Laachraoui was only publicly identified as a Paris suspect on Monday and Brussels was targeted less than 24 hours later.

He had left Belgium for Syria in February 2013 where he received terror training before returning to Europe posing as a refugee from the conflict.

He was picked up in Budapest in September last year by Abdeslam and taken to Brussels where he is believed to have set up bomb-making factories and prepared for the Paris attacks.

He was also given the fake identity of Soutane Kayal which he then used to move across Europe. It was during this journey from Budapest to Brussels that police stopped the car but were waved on.

Travelling with him was Mohamed Belkaid​, 35, who was killed in a police raid last Tuesday and led to the eventual capture of Abdeslam and another suspected terrorist.

Under his fake identity, Laachraoui rented a house in Auvelais, near the central Belgian city of Namur​, used by some of the Paris killers and at another suspected hideout in the Rue Henri Berge in Schaerbeek.

In February 2015 he left for Syria, where his nom de guerre​ was Abou Idriss.

Laachraoui's DNA was later found on two of the suicide vests used in the Paris attacks. It was also found in the addresses raided in Schaerbeek and Auvelais.

He is believed to have been a key figure in the Paris plot and directed Abdeslam on how to assemble the bombs.

Police suspect the Paris attackers were in phone contact with Laachraoui and Belkaid on the night of the massacre, sending Belkaid the text message "we're off, we're starting" seconds before launching the attack on the Bataclan concert hall. Their mobile phone signals were tracked to Schaerbeek.

Laachraoui was already suspected of recruiting others to fight in Syria and was linked to Abdelhamid Abaaoud​ – the Belgian mastermind behind the Paris attacks.

He was recently tried in absentia for involvement in a network of Belgians who left for Syria in which the prosecutor called for a 15-year prison sentence for persuading several of his friends to join the ranks of IS. The verdict is due in May.

"He is an old customer among foreign fighters," one investigator told La Libre Belgique.

He went to school at the l'Institut de la Sainte-Famille d'Helmet in Schaerbeek.

Staff at the school said: "He did all his secondary schooling here. He had a totally normal school l life, he never repeated a year and left the school in 2009. He was a boy who had no problems."

According to La Libre Belgique, he then went on to complete his first year in higher studies in electromechanics, where he obtained "satisfactory" marks, suggesting he was technically minded.

The Brussels bombers are thought to have used an explosive known as acetone peroxide and TATP and nicknamed "Mother of Satan" that was also used in the July 7 attacks in London in 2005, as well as Paris. It has been used in numerous terrorist bombs and suicide attacks.

Terrorists particularly like it because it does not contain nitrogen, and therefore can pass through security screening devices that rely on a nitrogenous presence to detect explosives.

The explosive gets its nickname because it is extremely sensitive to shock, heat and friction, and frequently ends up killing the bomb-maker during the manufacturing process.